1. Field of the Invention
The present application relates to wafer metrology, and more particularly to approximating eigensolutions for use in determining the profile of a structure formed on a semiconductor wafer.
2. Related Art
Optical metrology can be utilized to determine the profile of structures formed on semiconductor wafers. In general optical metrology involves directing an incident beam at a structure and measuring the resulting diffraction beam. The characteristics of the measured diffraction beam (i.e., a measured diffraction signal) is typically compared to pre-determined diffraction signals (i.e., simulated diffraction signals) that are associated with known profiles. When a match is made between the measured diffraction signal and one of the simulated diffraction signals, then the profile associated with the matching simulated diffraction signal is presumed to represent the profile of the structure.
In general, the process of generating a simulated diffraction signal involves performing a large number of complex calculations, which can be time and computationally intensive. The number and complexity of the calculations increases for structures having profiles that vary in more than one dimension.